Pipeline (C-34) · Lesson 3 of 6

Thrust Block Sizing at Bends, Tees, and Dead Ends

"Thrust Block Sizing at Bends, Tees, and Dead Ends" is lesson 3 of 6 in the Pipeline (C-34) Learning Hub. It opens with "Why and Where Thrust Blocks Go" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Why and Where Thrust Blocks Go
  2. What Sets the Block Size
  3. Read the Thrust From Table 2 and Scale It to Your Pressure
  4. Divide by the Safe Bearing Load of the Soil
  5. Have the Blocks in Place Before You Test

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Scale to the actual pressure" through "Check the schedule".

What you'll be able to do

  • Explain the main idea of "Thrust Block Sizing at Bends, Tees, and Dead Ends" in plain language you can use on the exam.
  • Apply this step with confidence: "Read the Thrust From Table 2 and Scale It to Your Pressure".
  • Apply this step with confidence: "Divide by the Safe Bearing Load of the Soil".
  • Apply this step with confidence: "Have the Blocks in Place Before You Test".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Thrust Block Sizing at a 90 Degree BendT = 27,150 lb(resultant)Plan view, 12-in 90° bendtest pressure 150 psiFLOWunbalancedthrust at bendBlock bears onundisturbed sand,2,000 psfTABLE 2: THRUST, lb AT 100 psiNPS90°45°22.5°Dead end/tee88,3004,5002,2905,8601218,1009,8005,00012,800IAPMO IS 33 METHODTable 2, 12-in, 90° bend:18,100 lb at 100 psiPressure ratio:150 / 100 = 1.5Thrust T = 1.5 x 18,100T = 27,150 lbTable 4, sand = 2,000 psfArea = T / bearing= 27,150 / 2,000= 13.58 sq fte.g. a 3.0 ft x 4.6 ftface = 13.8 sq ftBearing face of at least 13.58 sq ft against undisturbed 2,000 psf sandConcrete blocks: wait at least 24 h before the pressure test (IS 33 §6.1)California Plumbing Code 2025, Appendix I: IAPMO IS 33-2019 Tables 2 and 4 and §5.2 (not mandatory unless adopted)CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Why and Where Thrust Blocks Go

Rubber-gasketed pipe joints do not hold themselves together under internal pressure, so at a bend the water pushes the fitting toward the outside of the turn. IAPMO Installation Standard IS 33-2019, printed in Appendix I of the 2025 California Plumbing Code (CPC), says thrust blocks prevent separation of joints and pipe movement by transferring the resultant thrust force at a bend to the undisturbed soil behind the block. Section 4.1 requires them in rubber-gasketed lines at changes in direction (tees and bends), changes in size (reducers), stops (dead ends), and valves where thrusts may be expected. The 2025 Caltrans Standard Specifications section 20-2.08C(1) likewise requires a thrust block at each change in direction on a main supply line and at a terminus run.

Step 2: What Sets the Block Size

Section 5.1 of IS 33 says block sizes are based on three things: the maximum line pressure, the pipe size, and the safe bearing load of the soil. Note the scope: the standard covers ductile iron pipe with elastomeric gasketed joints and PVC pipe for cold-water building supply and yard piping, and the CPC prints it in Appendix I as a reference that is not mandatory unless a state agency or the adopting ordinance references it. Large public-works mains are designed by the engineer, but this method is how the thrust-to-bearing-area logic works.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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